The present invention relates to a feature-region extraction method and a feature-region extraction circuit and, more particularly, to a feature-region extraction method and a feature-region extraction circuit which are capable of extracting a feature-region of an image for processing the image according to its feature at a pre-processing portion of an image coding device and which are intended to be used, for example, in video telephone systems, video conference systems and so on.
At a motion picture coding and transmitting device for a video telephone, since any transmission line has a limited or finite capacity to transmit a number of bits at a time, it cannot allocate enough bits to an image information. Accordingly, a decoded image may have an impaired picture quality with a mosquito-like distortion, a block distortion and the like. On the other hand, most of the images to be transmitted include an important portion and less important portions. On the basis of this fact, a method is proposed that reduces the subjective deterioration of an image quality by keeping the important portion free from the ill effects of deterioration.
For example, in systems such as video telephone systems, the image of the face region is so important that such systems have been directed toward extracting the face region, preferentially allocating the amount of information transmitted and making the image quality of the face region better, aiming at improvement of the subjective image quality.
The prior art that is concerned with the present invention is disclosed in the publication "Color motion picture coding device featured by preferentially processing a face image" (Hideyuki Ueno, Image Information vol. Mar. 24, 1992, pp. 29-35). The face-region extraction method of the prior art described in the publication will be explained as follows.
The prior art first determines the difference between two neighboring frames of the motion picture inputted to get the interframe differential image and then divides the interframe differential image into rectangular regions. In the respective rectangular regions, the method makes a histogram representing the number of pixels, in which the interframe difference signal is larger than a certain threshold, on an H-axis. The method scans the histogram from the left to right, comparing its values with a certain threshold "th" to detect the left and right ends of the moving region.
The image screen is scanned from the upper left end until a first moving region is found. This moving region has a coordinate on a V-axis, which represents a position at the top of one's head. The image screen is then scanned from the left and the right by a certain range of lower image to determine the left and right ends of the moving region. And then, the outermost positions of the moving region on the H-axis represent the coordinates of the left and right ends of the face. The face length is determined by multiplying the face width by a predetermined coefficient ".alpha.". The prior art considers the thus determined image region as a face-region.
As mentioned above, the conventional feature-region extracting method estimates a face-region from an interframe difference signal representing the amount of motion. Consequently, if the image includes any other moving object than the face-region, the prior art method may erroneously take the object as a face part. Furthermore, the method can roughly extract a face-region only in the form of a rectangle, which contour may be emphasized when the regions other than the face are adaptively processed with a low-pass filter. This may impair the subjective image quality.